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The Treaty of Paris had established the 45th parallel as part of the northern boundary of modern-day New York and Vermont. Most of that portion of the boundary had previously been surveyed in the early 1770s, but the survey line was inaccurate. Since "Fort Blunder", an unnamed U.S. fort in what is now part of northeastern New York, had been constructed north of the actual 45th parallel, the United States wanted to follow the old survey line, and the Webster-Ashburton treaty incorporated this change, leaving the half-finished fort on U.S. soil. Following the signing of the treaty, construction was resumed on the site. The new project replaced the aborted 1812-era construction with a massive third-system masonry fortification known as Fort Montgomery.[1]

This treaty marked the end of local confrontations between lumberjacks (known as the Aroostook War) along the Maine border with the British colonies of Lower Canada (Quebec) and New Brunswick. The border was fixed with the disputed territory divided between the two nations. The British acquired the Halifax-Quebec road route which their military desired because it gave a wintertime connection from Quebec to New Brunswick and Nova Scotia. Portions of the border were adjusted so as to give the U.S. a little more land to the north. It also resolved issues that had led to the Indian Stream dispute as well as the Caroline Affair. The Indian Stream area was assigned to the U.S. The Webster–Ashburton Treaty failed to clarify ownership of Machias Seal Island and nearby North Rock, which remain in dispute today.[1] Additionally, the signing of the treaty put an end to several building improvements planned for Upper Canadian defense forts such as Fort Malden in Amherstburg, which as a result was later abandoned by the British government as it no longer served a defensive purpose.

The border between Lake Superior and the Lake of the Woods needed clarification because the faulty Mitchell Map used in the negotiations for the Treaty of Paris was inadequate to define the border according to the terms of that treaty. The ambiguity in the map and treaty resulted in Minnesota's Arrowhead region being disputed, and previous negotiations had not resolved the question. The treaty speaks of the border passing through "Long Lake", the location of which was unstated,[2] but the map showed the lake flowing out into Lake Superior near Isle Royale, which is consistent with the Pigeon River route.

To counter this western route, the U.S. advocated for an eastern route, used by early French explorer Jacques de Noyon in 1688, and the later a well-used fur traders' route after 1802. This way headed north from the lake at the site of Fort William up the Kaministiquia and Dog Rivers to Cold Water Lake, crossed the divide by Prairie Portage to Height of Land Lake, then went west by way of the Savanne, Pickerel, and Maligne rivers to Lake La Croix, where it joined the present border.[5]

The Mitchell map had shown both of those routes, and also showed the "Long Lake" route between them.[6] Long Lake was thought to be the Pigeon River (despite the absence of a lake at its mouth).[a]

The traditional traders' route left the Lake at Grand Portage and went overland to the Pigeon, up that river and a tributary across the Height of Land Portage, and thence down tributaries of the Rainy River to Lac La Croix, Rainy Lake and River, and Lake of the Woods. This is finally the route which was designated as the border in the treaty.[7]

Another clarification made in this treaty resulted in clarifying the anomaly of the Northwest Angle. Again, due to errors on the Mitchell Map, Treaty of Paris reads "... through the Lake of the Woods to the most northwesternmost point thereof, and from thence on a due west course to the river Mississippi ..." The Anglo-American Convention of 1818 defined the boundary about Lake of the Woods to the Rocky Mountains.

This 1842 treaty reaffirmed the border and further defined it by modifying the border definition to instead read as

"... at the Chaudiere Falls, from which the Commissioners traced the line to the most northwestern point of the Lake of the Woods, thence, along the said line to the said most northwestern point, being in latitude 49°23′55″ north, and in longitude 95°14′38″ west from the Observatory at Greenwich; thence, according to existing treaties, due south to its intersection with the 49th parallel of north latitude, and along that parallel to the Rocky Mountains ..."

The Webster–Ashburton Treaty failed to deal with the Oregon question, although the issue was discussed in negotiations.

Article 10 of the Webster–Ashburton Treaty identified seven crimes subject to extradition: "murder, or assault with intent to commit murder, or piracy, or arson, or robbery, or forgery, or the utterance of forged paper." It did not include slave revolt or mutiny. In addition, the United States did not press for the return or extradition of an estimated 12,000 fugitive slaves who had reached Canada.[8]

While agreeing to call for a final end to the slave trade on the high seas, Webster and Ashburton agreed to pass over the Creole case of 1841 in the Caribbean, which was then in contention. In November 1841, a slave revolt on the American brig Creole, part of the coastwise slave trade, had forced the ship to Nassau. Bahama officials eventually emancipated all 128 slaves who chose to stay in Nassau, as Britain had abolished slavery in its colonies, effective in 1834.[8] The U.S. initially demanded return of the slaves, then compensation. A settlement was made in 1855 as part of a much larger claims treaty of 1853, covering claims by both nations to 1814.

As a result of the Webster–Ashburton Treaty, the United States ceded 5,000 square miles (13,000 km2) of disputed territory along the Maine border, including the Halifax–Quebec Route, but kept 7,000 square miles (18,000 km2) of the disputed wilderness.[9] In addition, the United States received 6,500 square miles (17,000 km2) of land along the Minnesota-Canada border, which included the Mesabi Range.[9] Shortly after ratification of the Webster–Ashburton treaty, the Ojibwa nations about the south shore of Lake Superior ceded land to the United States in the Treaty of La Pointe. However, the news of the ratification of the international treaty did not reach either of the parties negotiating the land cession. The Grand Portage Band was mistakenly omitted from the Ojibwe treaty council. In addition, the Grand Portage Band was misinformed on the details of the Treaty of Paris; they believed that the border passed through the center of Lake Superior to the Saint Louis River, placing both Isle Royale and their band in British territory. The Treaty of Paris specifically notes Isle Royale to be in the territories of the United States. Consequently, the Isle Royale Agreement was signed between the United States and the Grand Portage Band in 1844 as an adhesion to the Treaty of La Pointe, with other Ojibwa tribes reaffirming the treaty.

In order to make the controversial treaty more popular in the United States, Webster released a map of the Maine-Canada border, which he claimed had been drawn by Benjamin Franklin. It showed British ownership at the time of the area the United States claimed and largely received under the Treaty.

^On the La Vérendrye Map, series of lakes are shown, of which "Lac de Sesakinaga" (Saganaga Lake), a Height of Land, "Lac Plat", "Lac Long" and Grand Portage are shown in relative equidistance from each other, thus alluding to Mountain Lake or Arrow Lake as "Lac Long", all long lakes on the Pigeon River route.

LeDuc, Thomas (December 1964). "The Webster–Ashburton Treaty and the Minnesota Iron Ranges". Journal of American History. 51 (3): 476–481. JSTOR1894897. (shows the value of the iron range was not known when the treaty was drawn)